1. Field of the Invention
The present invention relates to a fiber-type wavelength converting device that produces a second harmonic output light generated by Cherenkov radiation. More particularly, the fiber-type device includes a nonlinear optical crystal core and a cylindrical cladding of a glass-like material. The generated second harmonic signal has a high signal-to-noise (S/N) ratio.
2. Description of the Prior Art
Nonlinear optical effects are phenomena wherein light that is incident upon a certain medium produces polarization proportional to quadratic or high terms of the intensity of the electric field of the light. These phenomena cause such events as the generation of second harmonic waves.
Devices having media capable of generating nonlinear optic effects in incident light are called "nonlinear optical devices". Inorganic materials such as KH.sub.2 PO.sub.4 and LiNbO.sub.3 are commercially available as nonlinear optical devices. Recently, much attention has been focused on organic materials, such as 2-methyl-4-nitrileaniline (MNA), due to their high nonlinear optical constants.
Nonlinear optical devices used to generate harmonics are designed as an optical integrated circuit made of a substrate with a lightguide formed thereon through which light propagates. The lightguide is covered with an overlay. In order to "pick up" the second harmonic generated in the lightguide, a waveguide path must be constructed to accommodate the phase velocity of the second harmonic propagating therethrough. That is, the waveguide path must be phase-matched with the second harmonic. The simplest way to do is to use Cherenkov radiation.
The principle of Cherenkov radiation is illustrated in FIG. 1. Light propagating through lightguide 111 generates a second harmonic at point A that is launched into both a substrate 121 and an overly 131 at an angle .theta.. If the equiphase plane of the generated second harmonic agrees with that of the second harmonic that is generated in the direction of angle .theta. at point B after a period of time, then a second harmonic is launched at point C in the direction of angle .theta.. Phase matching is automatically attained when the following condition is satisfied: EQU n.sub.s (2.omega.)&gt;n.sub.G (.omega.)&gt;n.sub.s (.omega.)
where n.sub.s (.omega.) is the refractive index of the substrate with respect to the fundamental wave, n.sub.G (.omega.) is the effective refractive index of the lightguide with respect to the fundamental wave, and n.sub.s (2.omega.) is the refractive index of the substrate with respect to the second harmonic wave.
It is further known that the Cherenkov radiation can be realized not only by a harmonic generating device in an optical integrated circuit, but also by an axially symmetric fiber-type wavelength converting device, the core of which is made of an organic material having a large nonlinear optical constant.
Depicted in FIG. 2 is a fiber-type wavelength converting device 9 including a core 10 constructed of an organic nonlinear optical crystal with a second-order nonlinear optical effect and a cladding 11 typically made of glass. Cladding 11 does not have any second-order nonlinear optical properties. In order to launch a second harmonic from the core into the cladding as shown by arrow 12, the following condition need be satisfied; EQU n.sub.cl (2.omega.)&gt;n.sub.co (.omega.)&gt;n.sub.cl (.omega.)
where n.sub.cl (2.omega.) is the refractive index of cladding 11 with respect to the second harmonic, n.sub.co (.omega.) is the refractive index of the core 10 with respect to the fundamental wave L1, and n.sub.cl (.omega.) is the refractive index of the cladding 11 with respect to the fundamental wave L1.
Since the second harmonic that is generated is launched from cladding 11 at a given divergence angle, the second harmonic beam emerging from end 9a of fiber-type device 9 produces an annular pattern as shown by 14. The fundamental wave is launched from the core at a small angle of divergence and produces a circular pattern indicated by 13.
The fiber-type device 9 is formed into a desired length by cutting the cladding and the core using a fiber cutter. However, the end face 9a is usually not cut flat as shown in FIGS. 3(a), 3(b) and 3(c). Hence, the emerging second harmonic wave is distorted.
In order to rectify this problem, it is possible for one to attempt to grind and polish the cut end surface 9a. However, in the current state of the art, the outside diameter of cladding 11 can not be greater than 300-500 .mu.m. Due to insufficient strength, many of the fine fibers will be broken during grinding and polishing, thereby decreasing the fiber yield.
An alternative to the polishing and grinding process is to fix fiber 9 within a ferrule using an adhesive and to grind and polish fiber 9 before cutting. If the entrance end face of the fiber is ground and polished, the nonlinear optical crystal core will collapse, thereby making admission of laser light, the usual source of the fundamental wave, impossible. Thus, the exit end face must be ground and polished with the entrance end being cut. This approach has been problematic since it has been difficult to cut the fiber to a desired length of about several millimeters with the fiber device being fixed within the ferrule.
It is also known in the art to use the fibers described above in an apparatus developed so as to be capable to reading information from pits on an optical disk. FIG. 4 illustrates a typical arrangement for such a use. A light beam issues from semiconductor laser 81 and is focused to a point by passing through collimator lens 82 and focusing lens 83. The light beam is focused to a point on a fiber-type wavelength converting device 84. At the exit end of device 84, the fundamental wave emerges from the core while the second harmonic emerges from the cladding. The apparatus under consideration utilizes the second harmonic that emerges from the cladding. The second harmonic, which issues at a predetermined angle of divergence, passes through conical prism 85 so that the entire second harmonic is collimated. The collimated beams pass through a polarizing beam splitter 86 and the optical path of the beams is then bent by mirror 87. The beams then pass through quarterwave plate 88 and focusing lens 89 focuses the beams to be incident on optical disk 90.
Light that is reflected from a pit on disk 90 is again collimated by focusing lens 89. The reflected light passes through quarter waveplate 88 and is reflected by mirror 87. The reflected beams optical paths are further bent by polarizing beam splitter 86. Lens 91 focuses the beam so as to have it incident an photodetector 92.
Although the fundamental wave issuing from the exit end of the core of fiber-type wavelength converting device 84 is also incident on conical prism 85, the fundamental wave is not collimated by prism 85 as prism 85 is designed so as to collimate only the wavefronts of the second harmonic as these form through a given angle of divergence. Even if the fundamental wave passes through polarizing beam splitter 86, mirror 87, quarter waveplate 88 and focusing lens 89, it is not focused to a point on disk 9.theta.. Rather, the fundamental wave will illuminate a certain measurable area on disk 90 indicated by dashed line 93 in FIG. 4. The fundamental wave reflected from disk 90 is focused by lens 91 and directed into photodetector 92 like the second harmonic.
In order to increase the S/N ratio of the signal detected by photodetector 92 in the above-described apparatus, it is desirable to improve the intensity ratio of the second harmonic to the fundamental wave detected by photodetector 92. To do this, it has been common to insert wavelength filter along the optical path. However, to select the second harmonic by sole use of wavelength filters, a plurality of filters need be superposed on one another and the intensity of the inherently weak second harmonic is further reduced. Further, extra space is required for such wavelength filters.
Noting that the S/N ratio of the signal detected at photodetector 92 is approximately equal to the ratio of the intensity of the second harmonic to the fundamental at pits of disk 90, it is clear that to improve the S/N ratio at detector 92, it is theoretically possible to increase the intensity of the second harmonic at pits on disk 90, and to decrease the intensity of the fundamental over the area defined by dashed line 93. However, this is much more difficult to accomplish in practice than in theory.
The ratio of the second harmonic exiting fiber-type device 84 to the power of semiconductor laser 81 ("percent conversion to harmonic") is determined by the construction of fiber-type device 84. The light that emerges therefrom is condensed by conical prism 85, thus the quantity of the second harmonic on the surface of the optical disk 90 will be constant if a spot smaller in diameter than each pit on the disk is produced. Accordingly, one practical way to enhance the S/N ratio of the signal detected by photodetector 92 is to lower the illumination intensity of the fundamental wave of the surface of disk 90.
In order to lower the illumination intensity of the fundamental wave on the surface of the optical disk 90, it is advantageous to ensure that the angle of divergence of the fundamental beam emerging from fiber-type device 84 is increased as much as possible, thereby diffusing the beam as much as possible. It is possible to increase the angle of divergence by providing a greater difference in refractive index between the core and the cladding with respect to the fundamental wave. However, the fundamental wave is spherical and always contains a beam that emanates at a angle of divergence coincident with that of the second harmonic. Since this beam has coherent wavefronts, it is converged to twice as large spot as is the second harmonic wave before returning to the photodetector. This makes it impossible to achieve a significant improvement in the S/N ratio.
Further, in the above fiber-type device 84, the organic nonlinear optical crystal of the core 10 sublimates or changes its quality at the end of the device at which the optical crystal contacts an ambient gas such as air. Thus, the device has such disadvantages that wavelength conversion efficiency and input coupling efficiency of the fundamental wave are remarkably decreased and its optical loss increases, as a time lapses.
In order to solve the problems, Japanese patent application laid-open Nos. 79032/1990 and 79033/1990 disclose a method in which a thin film is formed on both ends of a fiber by a spin coat method etc. However, in case that the thin film is formed on the exit end face by the method, it is difficult to make the film thickness uniform. Accordingly, the method has a problem that aberration occurs on the wavefront of the second harmonic.